Apple MA434Z/A User Manual - Page 420

In the following example, MDiv and MMult nodes are added to color correct a 3Drendered element. Again, you can alternatively omit the MMult, and enable preMultiply in the Layer-Over node. Color correcting Result For an example of color correcting premultiplied elements, see Tutorial 3, "Depth Compositing" in the Shake 4 Tutorials. Combining Images in Other Ways Shake also has a set of mathematical and Boolean layering operators. The IAdd, IDiv, IMult, ISub, and ISubA nodes add, divide, multiply, or subtract two images together. The "I" stands for "Image." The second subtracting node, ISubA, returns the absolute value of the image. If you place a dark gray image in the first input (value .2, .2, .2), and then a white image (1, 1, 1) in the second input, the ISubA operation, it returns a light gray image (.2 - 1 = -.8, take the absolute value of, returning .8, .8, .8). This is a quick way to compare two images. The more flexible tool for generating difference mattes is the Common node, which is used to isolate common or different elements between two images. The other mathematical operators are Min and Max, which return the lower or higher values of two images, respectively. The Boolean operators Inside, Outside, and Xor are also useful for masking images: • Inside places the foreground image inside of the background alpha. • Outside places the foreground image outside of the background alpha. • Xor reveals only areas without a common alpha area. Shake contains several effect operators: • ZCompose for Z-based compositing • Screen, to composite light elements and preserve highlights • Atop, to place an element only on the foreground image. (For example, you can use the Atop node to place a smoke element over a CG character, matching smoke in the background plate.) 420 Chapter 15 Image Processing Basics